The effect of focused ultrasound on the electrochemical passivity of iron in sulfuric acid

Focused ultrasound was used to study passivity of pure iron in 2 N H₂SO₄. Ultrasonic waves were used to depassivate a passive surface film and influence the subsequent repassivation process. A curved piezo-electric transducer produced high frequency (1.58 MHz) ultrasonic waves which created cavitation at the focal point. Acoustic focal intensities up to 3.4 kW cm^?2 were generated. Low frequency (20 kHz) ultrasound was produced with a commercial sonicator equipped with an exponential microhorn. At high focal intensities (above 1.5 kW cm^?2) a single (100 ms) pulse of ultrasound produced depassivation; at low intensities continuous ultrasonic exposure was required. In all cases, the induced depassivation was followed by precipitation of a metal salt film upon the metal surface before the oxide film formation.